A scanning electron microscope (SEM) is an apparatus which detects secondary electrons (SE) and backscattered electrons (BSE) released from a sample by irradiating the sample with an electron beam (hereinafter also referred to as a primary beam). The detected signal can be converted into an image or line profile to thereby observe, inspect or measure the sample. Electron microscopes are widely applied to measurement of semiconductor devices and measurement technique thereof must be applied to measurement of various patterns with miniaturization of semiconductor devices.
Recently, in particular, there is a need for observation and measurement of a hole structure having a high aspect ratio named a contact hole. The aspect ratio is a ratio of the depth and the diameter of a hole and a contact hole having the aspect ratio exceeding 30 is not rare. In order to observe the bottom of a contact hole having a high aspect ratio, the bottom of the hole must be irradiated with a primary beam and secondary electrons released from the bottom must be detected. However, in case of the hole having the high aspect ratio, there is a case where the secondary electrons released from the bottom collide with the side wall of the hole and cannot escape from the hole, so that it is difficult to observe the bottom of the hole.
In order to lead the electrons released from the hole bottom onto a sample, there is a known method in which the surface of the sample is charged positively.
Patent Literatures 1 to 5 disclose a method in which the surface of a sample is irradiated with a beam (first beam) to charge the sample surface and thereafter a beam (second beam) for detecting electrons for formation of an image or a line profile of a contact hole or the like is scanned. Since the first beam has the secondary electron emission efficiency η and the irradiation area larger than those of the second beam, the surface of the sample is charged positively. The sample being in the positively charged state is irradiated with the second electron beam and accordingly the electrons released from the bottom of the hole can be led to the outside of the hole to be detected with high efficiency.